Based on the provided data, we can see that the coin in question has a grade of 30 (BC+), indicating that it is in acceptable but not excellent condition.

Furthermore, in previous auctions, starting prices have ranged from 1 to 25 euros, with estimated prices ranging from 9 to 40 euros. The average closing price has been 26 euros.

Given that the coin has been auctioned 5 times in the past and has sold on each occasion, we can infer that there is interest in it among collectors.

Taking into account these data points, as well as the fact that the coin will be auctioned in euros, and considering that the average closing price has been 26 euros, my estimate for the maximum purchase price would be around 35-40 euros. This is considering that the starting price is 12 euros and the estimated price in the last auction was 30 euros.